Abstract

We have investigated the carrier dynamics in multilevel intermediate-band solar cells (IBSCs) by solving the Poisson equation, the continuity equations of electrons and holes, and the balance equation of IBs self-consistently. The efficiencies of 6-level IBSCs have stronger dependence on the doping concentration than those of 3-level IBSCs. For non-optimal doping conditions under 1 sun, the efficiencies of 6-level IBSCs can be inferior to those of 3-level IBSCs and even single junction solar cells (i.e., 2-level IBSC). The reasons for this are that multiple IBs in 6-level IBSCs limit their ability to produce currents and the combinations of the energy bandgaps are not optimized for doping concentrations. On the other hand, at around half occupation of electrons in the IBs, the energy conversion efficiencies of IBSCs are maximized under any sun concentrations. The efficiency of 6-level IBSCs has a maximum (66% under 1000 suns) approaching the thermodynamic upper limit, which is similar to the case of 3-level IBSCs. These results indicate the importance of optimizing the doping concentrations in the IB regions of the 6-level IBSCs.

Received 28 February 2013Accepted 06 June 2013Published online 26 June 2013

Acknowledgments:

The authors would like to thank Professor S. Kako, Professor M. Kitamura, and Professor S. Iwamoto for their useful discussions and Professor K. Tanabe and Dr. M. Holmes for their invaluable comments on our manuscript as well as useful discussions. The authors also would like to acknowledge Dr. A. Takahashi, Mr. Y. Tomomura, Mr. S. Aomori, and Dr. M. Izumi for their encouragement and support to this work. This work was supported by Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.